smp, generic: introduce arch_disable_smp_support() instead of disable_ioapic_setup()

Impact: cleanup

disable_ioapic_setup() in init/main.c is ugly as the function is
x86-specific. The #ifdef inline prototype there is ugly too.

Replace it with a generic arch_disable_smp_support() function - which
has a weak alias for non-x86 architectures and for non-ioapic x86 builds.

Signed-off-by: Ingo Molnar <mingo@elte.hu>
diff --git a/arch/x86/kernel/apic.c b/arch/x86/kernel/apic.c
index 85d8b50..a04a73a 100644
--- a/arch/x86/kernel/apic.c
+++ b/arch/x86/kernel/apic.c
@@ -1138,9 +1138,7 @@
 	int i, j;
 
 	if (disable_apic) {
-#ifdef CONFIG_X86_IO_APIC
-		disable_ioapic_setup();
-#endif
+		arch_disable_smp_support();
 		return;
 	}